Tyre Rotation for Maximum Distance

A car has two front tyres and two rear tyres. Front tyres wear out after $x$ km, and rear tyres wear out after $y$ km (with $x \neq y$ in general). You can swap the front and rear tyres at any point during the journey. What is the maximum total distance the car can travel before any tyre is completely worn out? At what point should you perform the swap?